1. Lecture 3
Concepts of OOP
Object Oriented Programming
Eastern University, Dhaka
Md. Raihan Kibria

2. Difference between a Class and an
Object
 A class
 A class is a template
 Definition of a class is only given during design/coding
time
 A class can be compared to a struct in c
 An object
 An object is an instance of a class
 An object is made only during run time
 An object is made from the definition of a class

3. An example
Class definition:
public class A{
String code;
String name;
}
Code to make a new instance:
A a = new A();

4. Object/instance
 So far we have defined a class and written
code to instantiate it. But we still have not
created the object.
 Compile: javac A.java
 Run: java A.class
 Only now an object or instance of class A is
created.
 JVM creates the instance or object based
on class definition found in the byte-code

5. Some rules for creating class in
java
 A class can be defined in any file. However,
a public class must be in a file name that
matches the class name.
 Example of a public class:
public class Student{
String code;
String name;
}
file name: Student.java

6. Some rules for creating class in
java
 Example of non-public class:
class A{
String name;
String code;
}
Class B{
int age;
String program;
}
Fliename could be anything, such as
Example.java

7. Some rules for creating class in
java
 Please note that many non-public classes
may reside in one file.
 But a file can have only one public class.
 A file can have one public class and many
non-public classes.

8. Packages
 Packages are folders, roughly speaking.
 Typically, “package” is the first line in any
java program.
 Many java files can be placed under a
package.
 Package is a way to organize codes.

9. Example
package com.abc;
public class A{
String ...
}
This means the file A.java is under folder
com/abc

10. Example
public class B{
String ...
}
This means the file B.java is not under any
folder

11. Compiling packaged classes
Suppoer A.java is under com folder.
The first line of A.java is:
package com;
 To compile issue command
javac com/A.java
 The output A.class file will be produced
under com folder
 To run issue command:
java com.A

12. Compiling non-packaged classes
 Suppose B.java has no package definition
 To compile issue command:
javac B.java
To run issue command
java B

13. Compiling a package
 To compile all files under package issue
command:
javac com/*.java
 To compile multiple packages issue
command:
javac com/*.java com/abc/*.java